1. Field of the Invention
The present invention relates to magnetic recording media and a producing method thereof, in particular to a method of producing magnetic recording media which uses an extrusion coater to coat a continuously running flexible substrate (hereinafter referred to as xe2x80x9cwebxe2x80x9d) with a magnetic fluid and the magnetic recording media produced thereby.
2. Description of the Related Art
The production of magnetic recording media, such as magnetic disk and magnetic tape, have been carried out by coating a web with a certain magnetic fluid, and an extrusion coater has been often used in such production. This extrusion coater coats a continuously running web with a magnetic fluid by ejecting the magnetic fluid having been supplied to a coating head thereof to an edge surface of the coating head through slits formed in the edge surface, while relatively pressing the web and the edge surface of the coating head against each other. The coating width of the magnetic fluid applied on the web is regulated by coating width regulating plates fitted into the lateral edges of each slit. This allows uncoated portions, that is, portions coated with no magnetic fluid to be formed on the lateral edges of the web. This uncoated portion is what is known as web selvage portion which is to be cut off finally, therefore it does not need to be coated with the magnetic fluid.
In the extrusion coater, though a web and the edge surface of its coating head are spaced at a fixed distance apart because a coating fluid intervenes between them, contact occurs between the two at the aforementioned uncoated portions (hereinafter referred to as selvage portion) of the edges of the web, this may cause scrapes in the web. Occurrence of scrapes in the web leads to trouble with quality of the produced magnetic recording medium, such as drop-out, due to the powder of the scraping chips. In addition, the powder of the scraping chips accumulates on the edge surface of the coating head and pushes up the web, leading to trouble with coating, for example, too thick a coal; is applied to the web.
With the increase in recording density of the magnetic recording media and with the decrease in recording layer thickness of the same, there have been increasing demands in recent years that a thinner and uniform magnetic coat should be applied to a web at a higher speed, and at the same time, the web used is becoming thinner and more delicate. This in turn makes more serious the problems, such as trouble with coating, for example, too thick a coat resulting from scrapes caused in the web during coating operation and fracture of the web; as a result, there have been strong demands to take measures to deal with the scrapes in a web.
As the measures to prevent scrapes from occurring in a web, there are disclosed in Japanese Patent Application Publication Nos. 61-257268 and 1-102736 methods in which physical contact between the web and the edge surface of the coating head is prevented from occurring by coating the selvage portions of the web with a solvent before starting coating with an extrusion coater.
There is also disclosed in Japanese Patent Application Publication No. 6-20271 a method which is an improvement of the above prior art methods. The method prevents scrapes from occurring in a web with the aid of an overcoatedly primed portion (the portion of a primer coat showing on both sides of a magnetic coat) which is formed by applying a primer fluid to the web, prior to applying a magnetic fluid thereto, in such a manner as to allow the primer coat formed to be wider than the magnetic coat subsequently formed. The overcoatedly primed portion which is to remain on the surface of the web in the exposed state, however, gives rise to a new problem during the post processing after a roll of the magnetic recording medium has been produced, because the primer coat is intended to increase the adhesion between the magnetic coat and the web, and therefore, it has tackiness. In particular, the magnetic recording medium is subjected to heat treatment in such a state that it is wound up into a roll so as to set the coat having been applied thereto and remove the thermal stress from the same. This causes the overcoatedly primed portion to be softened and stick to the back surface of the web, resulting in occurrence of sticking trouble, commonly known as blocking trouble, such that the coat in the neighborhood of the web selvage portion is stripped when unwinding the roll. The expansion of the stripped portion caused by the blocking trouble then causes trouble with recording medium quality, such as drop-out. As the measures to cope with this problem, in the coating method disclosed in Japanese Patent Application Publication No. 6-20271, Tg (glass transition temperature) of the primer fluid applied to the overcoatedly primed portion is raised so that it will not be softened by the heat treatment.
The coating method disclosed in Japanese Patent Application Publication No. 6-20271 has the following disadvantages, while it is an excellent method.
Specifically, as the level of demands for dimensional stability of magnetic recording media becomes higher with the increase in recording density of the magnetic recording media in recent years, it becomes necessary that the heat treatment described above be carried out at a higher temperature for a longer period of time. As a result, even with the primer fluid of which Tg has been raised, blocking may occur during the heat treatment; accordingly, raising Tg of the primer fluid is not a basic solution. Further, raising Tg of the primer fluid means that the kinds of primer fluids applicable are limited, which leads to an extreme decline in degree of freedom of selecting a primer fluid.
The present invention has been developed in view of the above-described circumstances, and has as its object the provision of a magnetic recording medium which enables not only getting rid of trouble due to scrapes occurring in the selvage portion of a web, but also preventing blocking trouble from occurring in heat treatment during post processing and a method of producing the same.
In order to attain the above-described object, the present invention is directed to a coating process in which a magnetic coat is formed by coating a continuously running web with a magnetic fluid ejected to the edge surface of a coating head through slits of the same, leaving the lateral edges of the web uncoated, while relatively pressing the surface of the web and the edge surface of the coating head against each other and a primer coat is formed on the web prior to forming the above magnetic coat on the same, and the process is characterized in that overcoatedly primed portions are formed on both sides of the above magnetic coat in such a manner that the primer coat becomes wider than the magnetic coat and a release coat is formed on the back surface of the web at least at the portions corresponding to the above overcoatedly primed portions.
Further, in order to attain the above-described object, the present invention is directed to a magnetic recording medium, comprising: a web; a magnetic coat formed on a surface of the web; a primer coat formed on the surface of the web prior to forming the magnetic coat on the surface of the web and having overcoatedly primed portions which are formed on both sides of the magnetic coat in such a manner that the primer coat becomes wider than the magnetic coat; and a release coat formed on a back surface of the web at least at the portions corresponding to the overcoatedly primed portions.
According to the present invention, a release coat is formed on the back surface of the web at least at the portions in contact with overcoatedly primed portions so that the overcoatedly primed portions will not stick to the back surface of the web, whereby the overcoatedly primed portions can prevent scrapes from occurring in the selvage portions of the web during the coating of the web with a magnetic fluid, in addition, the overcoatedly primed portions will not stick to the back surface of the web, due to the release coat, during the post processing after a roll of the magnetic recording medium is produced, for example, during the heat treatment of the web in the roll on which the above-described coats are applied, whereby blocking trouble can be prevented from occurring. Moreover, according to the present invention, it is not necessary to raise Tg of the primer fluid for use in forming a primer coat, whereby the kinds of primer fluids applicable are not limited.